The present invention relates to a device for managing the electrodes for electrical discharge machining (EDM) machine tools. These machines use tool electrodes intended to machine workpiece electrodes. The device comprises at least one magazine or rack that has a series of individual housings designed to accept and hold exchangeable elements consisting of the said electrodes each mounted on an electrode holder. The device comprises at least one changer equipped with at least one gripping member intended to move the exchangeable elements between the said individual housings and an attachment system situated on the machine, and vice versa.
Such devices, which are widespread in EDM, are generally equipped with a linear magazine (rack) or magazine in the form of a carousel, in which the exchangeable elements are locked in the individual housings; a manipulator arm transports the exchangeable elements between the magazine and an attachment system generally consisting of a chuck connected either to the head or to the table of the EDM machine; a numerical control system controls the movements, the stoppings in reference positions and synchronizes the lockings/unlockings on the arm, the head or the table. There are a number of possible movements depending on a particular sequence and for example:
the arm moves into a precise position situated under the magazine so that the exchangeable element is unlocked from its grip in the magazine and locked onto the arm,
the arm moves into a reference position facing the head so that the exchangeable element is unlocked and locked onto the arm,
the machining head performs a movement to release the exchangeable element locked onto the arm,
the arm performs a movement to release the exchangeable element locked to the head, etc.,
in certain relatively slow devices it is the machining head which performs the movements needed for transferring the exchangeable elements,
in other, more high-speed devices, a pivoting arm manipulates two exchangeable elements simultaneously, one which goes into the head, and the other which returns to the magazine.
The devices for changing elements used commonly on die-sinking electrical discharge machine tools are relatively slow programmable devices, but are compatible with EDM which has admittedly acquired a reputation in the eyes of the public as being a slow process. The need to accelerate the rates of tool change has arisen belatedly because, in most cases, the overall profitability of the machining process is not found to be appreciably affected by tool change speed. Once a reserve of more than ten exchangeable elements becomes necessary, the magazines and changers available for electrical discharge machining become very bulky and can take up as much floor space as a second machine. That is because they have been built to correspond to the standard attachment devices of electrode holders commonly used in EDM, which conventional devices are now in very widespread use with users; these attachment devices require the exchangeable elements to be grasped, stored and locked in a vertical position. In the case of a circular magazine, this requirement dictates a vertical axis of rotation and a substantial diameter, hence a great size. A circular magazine with a horizontal axis would, however, allow a great many exchangeable elements to be stored in a minimum amount of floor space.
The attachment devices in question were developed in particular for electrical discharge machining so as to guarantee precise repositioning of the exchangeable elements particularly in the machining head of the electrical discharge machine. Electrical discharge machining is known for its working environment which is highly contaminated with fumes and particles that have arisen from the cracking of the dieletrics, which particles are liable to adhere to the mechanical interfaces compromising the precision of repositioning operations. That is a significant reason why devices equipped with conical tapers have had to be abandoned because of the excessively large bearing areas that have to come into contact with one another. These special attachment devices are described, inter alia, in the following patent documents: EP 0 255 042, EP 0 285 557, EP 0 997 220, WO 99 33 601, U.S. Pat. No. 5,634,757.
With all manufacturers, they generally comprise a narrow drawbar attached axially to the electrode holder; the device forming the connection between the bar and the electrode holder is preferably designed to allow misalignment between the axes of the bar and of the electrode holder so that the bar does not transmit any torque to the electrode holder. Such a result is achieved for example by means of a toggle joint.
Near to the end of this drawbar, a retaining element in the form of a circular groove collaborates with a locking system consisting of balls and springs situated in the head or the table of the machine. These attachment devices comprise two sets of four surface-ground bearing faces facing one another, one set on the head or the table, and the other set on the electrode holder and which create a plane of contact perpendicular to the axis of the system of attachment to the head or to the table. The bearing faces situated on the head or on the table are each equipped with pneumatic orifice at their centre. A powerful jet of compressed air drives the impurities away at the moment that the opposing bearing faces come to press against one another. In addition, these bearing faces are of small surface area so that the contact pressure is enough to crush any impurity that may have remained adhered thereto. Depending on the manufacturer, a group of four bearing faces on protruding bosses are attached either to the electrode holder or to the machining head, for example.
The drawbar needs to be narrow enough that it can be housed between the four bearing faces and also allow a fork of the changer to be inserted on each side of this same bar without touching the bosses.
These attachment devices further comprise elastic pieces generally in the form of plates which have the special feature of being deformable in the direction of the axis of the machining head, but which offer a great deal of rigidity in the direction perpendicular to that same axis. Alignment elements generally in the form of pins shaped as pyramids made of a very hard material and attached to the machining head are set into these elastic pieces and force the electrode holder to position itself laterally with respect to the machining head 4 of the machine in a precise and repeatable manner. Because the force transmitted by the drawbar to the electrode holder is only an axial force, the positioning of the bearing faces and of the pins are not disturbed by parasitic stresses. The narrow drawbar also comprises in its middle two superposed grooves corresponding to two types of forks: changer forks and magazine forks. The narrow drawbar therefore on the one hand allows the electrode holder to be locked into the bore of the attachment system connected to the head of the machine or to the table and on the other hand allows it to be grasped and manipulated using a simple fork.
A number of suppliers of good repute such as EROWA, SYSTEM 3R or HIRSCHMANN for example, offer accessories to be incorporated into the head or into the table of the machine and ranges of electrode holders dedicated to electrical discharge machining. The principles of attachment, grasping, repositioning and locking used are similar from one supplier to another, although the suppliers are differentiated in terms of constructional details. As a result, users generally choose a single make and equip their entire holding of EDM machines with hardware from one single attachment system supplier, even if they possess electrical discharge machine tools of different makes. As a result, one and the same machine fitted with an attachment system of a given make can no longer accept electrode holders of a different make.
If such electrode holders are loaded symmetrically, kept vertical, suspended by their drawbar and handled at a sufficiently low speed, then this forks system is an acceptable way of holding the said electrode holders in position while they are being transported and stored.
Users of die-sinking electrical discharge machine tools frequently have to produce a great many electrodes of different shapes for machining a single component or a single mould. These electrodes have to be kept available in the rack or magazine throughout the machining process because they may be used several times each in order to produce cavities that are repeated at several points in the workpiece. As a result, because each cavity in isolation takes only a few minutes to machine, the time spent changing the electrode tends to become significant and the user does not feel free to choose the optimum machining sequence, that is to say the order that best suits him for using and reusing the various electrodes available in the magazine.
Further, during preparation for these complex machining sequences involving a great many electrodes, the operator wishes to carry out operations of presetting the electrodes or to perform a “dummy run” before beginning the actual machining operation proper, in order, for example, to eliminate any risk of incorrect positioning of an electrode on his electrode holder or of exchangeable elements being switched in the magazine. The entire exchange sequence is then run with the operator looking on and taking geometric measurements in the work space and checking that his program is running correctly. Each electrode is brought into position facing the cavity it is supposed to machine or into contact with a measuring sensor, but machining is not enabled during these operations. The operator obviously wants the dummy run and setup phase to be over as quickly as possible.
For these reasons a need has recently arisen in the field of electrical discharge machining to have available programmable controllers or management devices which are more rapid for changing the electrode holders. This is why the first thought was to accelerate the movements of the programmable controllers already available. Unfortunately, numerous incidents such as the dropping of exchangeable elements, incorrect gripping in the attachment system on the machine, jamming, collisions, were noted. These incidents were caused by the fact that the forks of the changers take hold of the exchangeable elements via the very narrow drawbar described hereinabove and such attachment unfortunately does not provide sufficient rigidity under these new conditions because of the clearance and the small diameter of the gripping groove around the said drawbar. During the rapid rotary movement of the changer, the electrode holder, weighed down by an electrode which is sometimes heavy and unbalanced, is subjected to dangerous oscillations detrimental to the reliability of the installation.
In spite of these constraints, it would be desirable if a new concept for electrode management were to give the user the option of reusing his existing stock of electrode holders irrespective of their make; that is to say without having to renew it or even modify it because that would prove prohibitively expensive for him; specifically, certain users have a considerable stock of electrode holders to which certain electrodes are permanently fixed and secured with a view to carrying out several machining operations on several different machines and over a lengthy period, that is to say one lasting several weeks or months. It would also be desirable for such an electrode management device to be capable of manipulating several if not all makes of EDM-specific electrode holder available on the market.